Small features on semiconductor chips, packages, and wafers may be measured or inspected with scanning laser beam triangulation equipment. Such equipment has been previously described in the literature and in patents, such as U.S. Pat. No. 5,859,924, issued to Liu et al., entitled “Method and System for Measuring Object Features,” the disclosure of which is incorporated herein in its entirety by reference thereto.
Laser triangulation measuring equipment generally operate by projecting, with a laser beam having a wavelength centered at approximately 830 nm (infrared (IR) radiation), a light spot having a preset spot size onto the surface to be examined, e.g., from a laser projection “gun” that may be mounted normal to the surface being examined. A light detection unit including a lens and a light detecting element or “camera,” such as a CCD or CMOS imaging chip or a position sensing device (PSD), e.g., of silicon, at an offset angle to the projection axis may observe the position of the laser spot in its field of view and output a signal describing the angle at which the spot appeared in the field of view. The range to the object can be computed from the angle information when the distance between the laser projection axis and the light detection unit is known. The offset angle between the laser beam and the line of sight of the light detection unit is often referred to as the “triangulation angle.” Based on which part of the detector the light reflected from the imaged object impinges, the height or “z-component” of the object at the point at which the light spot impinges upon the object may be determined.
To get high accuracy in the depth or range measurement, it may be required that small changes in depth be discernible on the light detection unit. This may require that the change in angle due to a small change in depth cause a significant change in position of the image of the projected light spot on the light detection unit which, in turn, may require that the magnification between the object and the light detection unit be near unity or greater.
It may be advantageous, particularly when imaging a highly reflective object, that the image of the laser spot be sharply focused on the light detection unit regardless of the height variation of the surface being imaged. This may be accomplished, e.g., by focusing the light detection unit on the projected light beam rather than on the surface to be observed by tilting the lens and the light detecting element with respect to the detection unit's optical axis as is described in U.S. Pat. No. 4,238,147, issued to Stern, entitled “Recording Images of a Three-Dimensional Surface by Focusing on a Plane of Light Irradiating the Surface,” the disclosure of which is incorporated herein in its entirety by reference thereto. When the active detecting element is a PSD, it may be required for all reflections of the projected spot from nearby objects be eliminated. This may be made possible by the incorporation of a segmented PSD in the light detection unit as described in U.S. Pat. No. 5,554,858, issued to Costa et al., entitled “Segmented Position Sensing Detector for Reducing Non-Uniformly Distributed Stray Light from a Spot Image,” the disclosure of which is incorporated herein in its entirety by reference thereto.
Scanning laser beam triangulation equipment have two basic configurations. In one configuration, the laser beam is scanned in the plane formed by the laser beam and the triangulation angle. See, e.g., U.S. Pat. No. 4,627,734, issued to Rioux. In another configuration, the laser beam is scanned transverse to this plane, which is the most commonly used configuration, as discussed in U.S. Pat. No. 6,031,225, issued to Stern et al., entitled “System and Method for Selective Scanning of an Object or Pattern Including Scan Correction,” the disclosure of which is incorporated herein in its entirety by reference thereto. The laser beam may be scanned back and forth at a high speed rate in a sawtooth, triangular, or sinusoidal motion. Typical scan rates are between 4 and 8 kHz with 500 to 1000 points measured during each scan.
In both cases, a motion axis is usually provided that is orthogonal to the laser scan so that data may be gathered over an area of interest rather than just over a single line whose length corresponds to the length of the scan. The mechanical motion of the sensor relative to the object being inspected or measured may be achieved by moving either the sensor or the object along the mechanical axis. When scanning semiconductor parts in a tray, it is most common to physically move the sensor head containing the scanning laser beam triangulation assembly, rather than to move the object. The converse is true when scanning a semiconductor wafer. The laser beam may be scanned, e.g., via a mirror controlled by a galvanometer, via a rotating polygon with mirrored facets, via a chip with multiple micro mirrors as implemented by Texas Instruments and used for television displays, or via an acousto-optic deflector which is the most commonly used device. The acousto-optic deflector makes use of the fact that a fine pitch sinusoidal grating may be used to deflect a light beam and that such a grating may be induced in various transparent crystalline materials by energizing them with an electromechanical transducer that is electrically driven with a radio frequency (RF) voltage. Varying the RF frequency varies the grating pitch inversely. The resulting laser beam deflection is proportional to the RF frequency input to the device. Thus, a linear variation of the RF frequency into the device causes a linear angular deflection of the beam. This type of deflector, currently in wide use in laser scanning systems, has the advantage of not requiring any moving parts, but, when used to obtain large high speed deflections for a focused beam, may suffer from field tilt and curvature and coma due to uneven spot size across the scan due to a “walkoff” phenomena as outlined in U.S. Pat. No. 5,517,349, issued to Sandstrom, entitled “Process and a Device for Error Correction in Acousto-Optic Deflection of Light Beams, Particularly of Laser Light.”
In order to avoid shadowing caused by either the laser beam or the camera line of sight being blocked by nearby objects or due to the inspected part being located in a depression such as a shipping tray pocket, it may be necessary to limit the offset angle between the laser beam and the angle of the offset light detection unit. Often, the angle is selected to be between 20 and 30 degrees.
A continuing difficulty with current triangulation sensors used for surface and lead inspection in the semiconductor industry is the need to simultaneously obtain a very small diameter spot for high definition and a large measuring range in depth, or depth of field. This tension exists because improving one of the spot size or depth of field necessarily requires sacrifice of the other, e.g., increasing numerical aperture decreases spot size but greatly decreases depth of field. Conventional triangulation systems provide a lens aperture based on a balance between the need for a small spot size and a large depth of field. Consistently, given the need for a minimum depth of field of 1.5 mm in versatile semiconductor lead scanning equipment, spot size has not been reduced beyond 35 microns. Accordingly, there is a need in the art for a method and system of providing increased precision in triangulation equipment, i.e., smaller spot size, without loss of depth of field.
Another difficulty with current triangulation sensors is the range of optical signals the light detection unit may be required to handle in the case of a highly specular target object being imaged. For example, because solder balls are very shiny, depending on the slope of the portion of the solder ball that is illuminated by the laser beam, light from the impinging laser beam may be reflected from a solder ball directly away from the detection unit's optics (so that the detection unit receives almost no signal from which to detect the z component) or directly towards the detection unit's optics (so that the signal is so strong that it may damage the detection unit). The very small and very large signals associated with the variation in slope may create a requirement for handling a very large dynamic range of signals compared to that needed for a diffuse object. Accordingly, there is a need in the art for a method and system of reducing the dynamic range of optical signals that the light detection unit might be required to handle.